The present invention is directed to a device for determining the positional deviation of n points, n being a natural number, from their n disjunct reference positions, using an electromagnetic radiation source, imaging optics, and a photosensitive detector, the positional information being converted into information on intensity.
To image flat or curved printing forms, whether it be in a printing-form imaging unit, in a print unit, or in a printing press, arrays of light sources, typically lasers, are used. With the array, which is usually oriented perpendicularly to the straight lines defined by the optical axis of the imaging optics, one produces a number n of individual light beams, whose image points from light sources, such as laser diodes, formed through an optical lens system, are distributed over a surface of a plurality of millimeters times micrometers, situated for the most part on a plane or even straight lines, on the printing form. A point or image point is understood in this context to be both a mathematical point, as well as a multi-dimensional, limited surface. The image points of an individual beam usually have a diameter of several micrometers, and they are spaced apart by several 100 micrometers. Often, the printing form does not abut so as to be flat against the base, be it a flat or curved surface, because the base is soiled by powder dust, other dust particles, or the like. Rather, local bulges having a diameter of several millimeters can form. The imaging optics of the array, both those which are identical for all n beams, as well as the individual ones, are generally configured such that the reference positions of the image points, in other words, their desired position at a reference distance to the optical lens system, are substantially located in one plane. However, because of the bulges, it necessarily follows that image points of individuals beams lie in a plane other than the plane which is defined by the reference position and which is perpendicular to the straight line defined by the optical axis of the imaging optics. To achieve a desired imaging result at these locations in the image field as well, depending on the method employed, one must either change the optical power for the affected light sources in the array, or, however, particularly when the image points in the reference position are a question of the beam waist of the light source, one must shift the focus of the imaging optics, either by varying the object distance, the image distance, or by shifting the main planes of the imaging optics. In both cases, one must determine the position of the current image point with respect to its reference position, since this quantity is needed as an input value to calculate the required change in power or the required variation in the imaging optics. Typically, the result of a ranging or distance measurement of this kind is used to generate a control signal. A control signal can be produced, for example, by further processing a signal from a photosensitive detector, thus from a measurement of light intensity. Optical distance-measuring devices are used, in particular, in autofocusing devices.
U.S. Pat. No. 4,546,460 describes an autofocusing device for an optical system having a laser as a light source, a light-reflecting surface, and a photodetector having at least two photosensitive regions. The laser beam is converged through an objective lens and projected onto the light-reflecting layer. The laser light reflected off of the layer is projected through the objective lens and other optical components onto the surface of the photodetector. In response to displacement of the objective lens along the optical axis, the laser beam is deflected, and the pattern projected onto the surface of the photodetector moves in a specific direction. When the objective lens lies at a distance shorter than a predetermined distance from the light-reflecting layer, the pattern is formed on the first photosensitive region. When the objective lens is located at a distance greater than the second predetermined distance, the pattern is likewise formed on the first photosensitive region. When the objective lens is located at a distance greater than the first predetermined distance and shorter than the second predetermined distance from the light-reflecting layer, the pattern is formed on the second photosensitive region of the photodetector. From the determination of the position of the pattern, one can deduce the distance of the light-reflecting layer to the optical system. Moreover, the focus of the imaging optics can be shifted by shifting the objective lens.
A system of this kind has the drawback of only allowing the position of one single point to be determined with respect to a reference position, and one single focus to be shifted.
U.S. Pat. No. 5,302,997 describes, for example, an arrangement of photometric and range finding elements in an array for use in automatic focus control and automatic exposure measurement for an associated optical system. The arrangement has a two-dimensional, photosensitive element in the center and, on either side thereof, a linearly disposed number of photosensitive elements in an image field. A lens system is provided for projecting an image onto the arrangement. In this context, the photosensitive elements disposed in a linear array receive light from a fractional portion of the image field and are used to measure the intensity of the light received, while the two-dimensional photosensitive element is composed of a number of individual regions and is used to generate a signal for automatic focus adjustment.
Here again, the disadvantage of this arrangement is that only the position of one single point is employed in focus control. Although an array of photosensitive elements is provided for measuring intensity, the corresponding signals are only employed in automatic exposure measurement.
The described devices are not suited for determining the deviation of the position of n image points from their reference positions for the n light sources of an array, in particular from lasers, since the n image points cannot be spatially resolved, and only one signal is produced for the entire image field. Successively measuring n deviations or distances implies an n-fold measuring time. This is not acceptable for the desired purpose of the application, particularly with respect to a device for forming an image on printing forms.
An object of the present invention is, therefore, to provide a device for determining the deviation of the position of n points from their n disjunct reference positions, which will render possible high-speed measurements of the n deviations or distances.
The present invention provides a device for determining the positional deviation of n points (P), n being a natural number, from their n disjunct reference positions, using a source of electromagnetic radiation (1), imaging optics (2, 4, 9), and a photosensitive detector (10), with the positional information being converted into information on intensity. Substantially simultaneous or concurrent in time n signals are produced by the detector (10), each of the n signals being uniquely assigned to one of the n points (P).
The present invention also provides a method for determining the positional deviation of n points (P) from their n reference positions, n being a natural number, comprising the following steps: illumination of each individual one of the n points (P) using electromagnetic radiation; conversion of the positional information on points (P) into path information on the light radiation; conversion of the positional information into intensity information; and discriminating detection of the reflected light from at least two of the n points (8); wherein the method steps are carried out simultaneously or concurrently in time for all n points (8).
In the device according to the present invention for determining the deviation of the position of n points from their disjunct reference positions using a source of electromagnetic radiation, imaging optics, and a photosensitive detector, simultaneous or concurrent n signals are produced by the detector, each of the n signals being uniquely allocated to one of the n points. To this end, light emanating from a light source is radiated through a suitable imaging optics onto the surface of n points, and is at least partially reflected off of the surface of n points. The reflected light is directed through an appropriate imaging optics to a photosensitive detector. Depending on the intensity of the incident light, a signal is produced, typically in electric form. As a result, a measurement of n points or points of reflection can be advantageously taken within a specific time. Using the device of the present invention, one can achieve a high-speed and simple measurement and generation of n signals, which can be utilized to either regulate the intensity of the light source in an array that is employed in an imaging device, in particular for printing forms, or, however, to change the focal positions of corresponding imaging optics for the imaging device, including the array. A device of this kind can be implemented in compact form and, likewise, entails low costs, since only one source of electromagnetic radiation is used. At the same time, the position of n points or points of reflection can be determined with proper resolution.
One of the aims of the present invention is to facilitate a rapid, spatially-resolved detection of surface unevenness on a printing form to be imaged, in particular to create a device suited for converting information on the printing form""s surface unevenness into a directly or indirectly detectable change in the position of a light beam or of a region of a light beam.
In one preferred specific embodiment, the source of electromagnetic radiation is a single source which emits coherent or incoherent radiation and whose light, when passing through one part of the imaging optics, impinges upon all n points, whose positional deviation from their disjunct reference positions is to be determined. The photosensitive detector has a number n of mutually independent photosensitive elements. Assigned to each of the n, mutually independent photosensitive elements is exactly one point or point of reflection, whose positional deviation with respect to the reference position is to be determined. Here, it is a question, in particular, of a distance deviation. In other words, the imaging through a further section of the imaging optics, following reflection of the light off of the reflecting surface, in whose area the n points lie, is conceived such that the light reflected off of the region of one of the n points, clearly follows from one of the n, mutually independent photosensitive elements. The deviation in the position of one of n points from its reference position leads to a different light path than the path of the light reflected from the point, through the imaging optics, into the reference position. In this manner, positional information is converted into path information. At least one element is provided in the imaging optics for converting the path information for each light path through the imaging optics associated with one of the n points, into information on light intensity. Particularly beneficial in this regard is the use of an optical element having a positionally dependent transmission, whether it be continuously or discretely positionally dependent. In other words, the device of the present invention for determining the deviation of the position of n points from their n disjunct reference positions can also be described as a parallel-processing, optical distance-measuring device.
The device of the present invention for determining the deviation of the position of n points from their disjunct reference positions can be conceived such that an imaging optics is employed which emanates from a source of electromagnetic radiation and has a plane of symmetry that runs in parallel to the optical axis of the imaging device. Alternatively thereto, it can be advantageous for the device of the present invention to be conceived such that its imaging optics projects a collimated beam that is obliquely incident to the printing form, onto a detector. As a function of the displacement of individual regions of the printing form out of the focusing position, points of intersection between the illuminating beam and the printing form can assume different spatial locations. The reflected beam is projected such that the spatial information pertaining to one direction, typically the direction of the cylinder axis, is retained when the printing form is mounted on a rotationally symmetric element, and such that the spatial information pertaining to a direction perpendicularly thereto, defined by the position of the n points, is converted to information on intensity.